Direct chill (DC) casting is an example of a vertical semi-continuous casting process, which is used for the fabrication of cylindrical billets from non-ferrous metals such as aluminium and alloys thereof. An example of a direct chill metal casting apparatus is described for example in U.S. Pat. No. 4,598,763. DC casting processes may also be used for the fabrication of metal ingots.
A DC casting apparatus typically includes a plurality of water-cooled moulds, each having an open ended vertical passageway through which the liquid metal flows. As the molten metal passes through the water-cooled moulds it is cooled causing the peripheral region of the metal to freeze. The mould is usually quite short (typically 75-150 mm) and as the metal emerges from the lower end of the mould it is cooled further by water jets causing the remainder of the metal to freeze, thereby forming a cylindrical billet. The lower end of the billet is supported by a starting head (or dummy block), which is lowered gradually (typically at a rate of 50-150 mm/min) by a hydraulic ram. Liquid metal is supplied continuously to the mould until the hydraulic ram reaches its bottom position. Typically, billets produced by the DC process have a diameter of 50-500 mm and a length of 4-8 meters.
A DC casting system normally has a plurality of moulds, typically allowing 2-140 billets to be formed simultaneously. The moulds are supported by a steel casting table and are fed with molten metal through a metal distribution system. There are two principal designs of DC casting system: in the first design the flow of metal is controlled by a float and in the second design the metal flows into mould through a feeding device made of a refractory material. The present invention relates to the second design, which is often called a “hot-top” casting system.
In a typical hot-top casting system the metal distribution system includes a plurality of refractory distribution devices called “cross feeders” that contain the liquid metal and distribute it to the moulds as the billets are formed. The distribution devices are typically made of a ceramic refractory material such as Insural® 140 made by Pyrotek Inc., which has a low thermal conductivity in order to prevent rapid cooling of the liquid metal before it passes through the moulds. The ceramic material must also have good mechanical properties. However, it can be difficult to obtain an ideal balance of mechanical and thermal properties, as refractory materials that have a very low thermal conductivity are often mechanically weak, whereas mechanically strong refractory materials tend to have a much higher thermal conductivity. Therefore, a refractory material with sufficient mechanical strength may have a relatively high thermal conductivity.
This can cause a number of problems. First, over an extended period of time (typically months or years) heat transferred by conduction from the liquid metal through the refractory distribution device to the steel casting table can cause distortion of the table through thermal fatigue. Typically, this results in a phenomenon known as “crowning”, in which the table takes on a slightly domed shape with the centre of the table being higher than its edges. Second, loss of heat from the liquid metal as it flows around the distribution system can give rise to temperature differences in different parts of the distribution system, the metal typically being hottest near to the metal feed point and coolest in parts of the distribution system that are furthest from the feed point. This can cause problems with the casting process as the metal emerging from “hot” parts of the distribution system will freeze more slowly than metal from “cool” parts of the system, thus making it difficult to match the speed of the hydraulic ram to the freezing rate of the metal.
It is an object of the invention to provide a distribution device than mitigates one or more of the above problems.